Traditional medical diagnostic processes record X-ray image patterns on silver halide films. These systems direct an initially uniform pattern of interrogating X-ray radiation through a patient to be studied, intercept the consequently image-wise modulated pattern of X-ray radiation with an X-ray radiation intensifying screen, record the intensified pattern in a silver halide film, and chemically transform this latent radiation pattern into a permanent and visible image, called a radiogram.
Radiograms have also been produced by using layers of radiation sensitive materials to directly capture radiographic images as image-wise modulated patterns of electrical charges. Depending on the intensity of the incident X-ray radiation, electrical charges generated either electrically or optically by the X-ray radiation within a pixelized area are quantized using a regularly arranged array of discrete solid state radiation sensors. U.S. Pat. No. 5,319,206, issued to Lee et al. on Jun. 7, 1994 and assigned to E. I. du Pont de Nemours and Company, describes a system employing a layer of photoconductive material to create an image-wise modulated areal distribution of electron-hole pairs which are subsequently converted to corresponding analog pixel (picture element) values by electro-sensitive devices, such as thin-film transistors. U.S. Pat. No. 5,262,649 (Antonuk et al.) describes a system employing a layer of phosphor or scintillation material to create an image-wise modulated distribution of photons which are subsequently converted to a corresponding image-wise modulated distribution of electrical charges by photosensitive devices, such as amorphous silicon photodiodes. These solid state systems have the advantage of being useful for repeated exposures to X-ray radiation without consumption and chemical processing of silver halide films.
In systems utilizing a photoconductive material such as selenium such as the prior art described in U.S. Pat. No. 5,319,206 (FIG. 1), before exposure to image-wise modulated X-ray radiation, an electrical potential is applied to the top electrode 1 to provide an appropriate electric field. During exposure to X-ray radiation 10, electron-hole pairs (indicated as − and +) are generated in the photoconductive layer 3 in response to the intensity of the image-wise modulated pattern of X-ray radiation, and these electron-hole pairs are separated by the applied biasing electric field supplied by a high voltage power supply 12. The electron-hole pairs move in opposite directions along the electric field lines toward opposing surfaces of the photoconductive layer 3. After the X-ray radiation exposure, a charge image is stored in the storage capacitor 8 of the TFT array 7A. This image charge is then readout by an orthogonal array of thin film transistors 7 and charge integrating amplifiers 6. This type of direct conversion system has the distinct advantage of maintaining high spatial resolution more or less independent with the thickness of the x-ray converting photoconductive layer. However, currently, only very limited number of direct converting photoconductors can be used for commercial products. The most popular and technical matured material is amorphous selenium that has good charge transport properties for both holes and electrons generated by the x-ray. However, selenium having an atomic number of 34 has only good x-ray absorption in the low energy range, typically below 30 Kev. For higher energy x-ray, the absorption coefficient of selenium becomes smaller and smaller and therefore thicker and thicker layers of selenium are required for adequate x-ray capture. Since the complication and difficulty of fabrication of good imaging quality amorphous selenium is a strong function of the selenium thickness, successful x-ray imaging product so far are limited to lower energy x-ray application such as mammography, low energy x-ray crystallography, low energy non-destructive testing.
In systems employing a layer of phosphor or scintillation material to create an image-wise modulated distribution of photons which are subsequently converted to a corresponding image-wise modulated distribution of electrical charges by photosensitive devices, such as amorphous silicon photodiodes, the scintillation generated by the absorbed x-ray may undergo multiple scattering and therefore spreading before being detected by the photo-sensitive imaging device. Phosphor types can be chosen to contain higher atomic number molecules such as Gadolinium (atomic number 64), cesium (atomic weight 133), iodine (53), Lanthanum (57), Terbium (57), Barium (56) and etc. for higher x-ray absorption coefficient. However, because of the scattering of the scintillation, this type of indirect conversion x-ray detector has a high point spread function resulting with a lower spatial resolution in compared to the direct conversion type such as selenium.
It is therefore desirable to design a radiation imaging system that has adequate radiation absorption over a wide radiation energy range as well as retaining high spatial resolution regardless of the thickness of the radiation absorption media.